(1) Field of the Invention
This invention relates to a radiation detector having scintillators, a light guide and photomultiplier tubes arranged in the stated order and optically combined to one another, and to a method of manufacturing the radiation detector.
(2) Description of the Related Art
This type of radiation detector is used in a medical diagnostic apparatus such a PET (Positron Emission Tomography) apparatus or a SPECT (Single Photon Emission Computed Tomography) apparatus for detecting radiation (e.g. gamma rays) released from radioisotopes (RI) introduced into a patient and accumulated in a site of interest, and obtaining sectional images of RI distribution in the site of interest. The radiation detector includes scintillators that emit light in response to incident gamma rays released from the patient, and photomultiplier tubes for converting the light emitted from the scintillators to pulsed electric signals. An earlier radiation detector had the scintillators and photomultiplier tubes arranged in a one-to-one relationship. In recent years, a technique has been employed to combine a plurality of scintillators to photomultiplier tubes smaller in number than the scintillators. With this technique, positions of incidence of gamma rays are determined from power ratios of the photomultiplier tubes to enhance resolution. A construction of a conventional radiation detector will be described hereinafter with reference to the drawings.
FIG. 1 is a schematic view showing an outward appearance of a conventional radiation detector. FIG. 2 is a section taken on line 100—100 of FIG. 1. FIGS. 1 and 2 show an example disclosed in Japanese Patent Publication No. 06-95146 (1994). This radiation detector RDA includes a scintillator array SA, a light guide LA optically combined to the scintillator array SA, a plurality of (four in FIGS. 1 and 2) photomultiplier tubes K1, K2, K3 (not seen in the figures) and K4 optically combined to the light guide LA. The scintillator array SA is an aggregate of scintillators S divided by numerous light reflecting elements DA inserted peripherally thereof. The scintillator array SA may be surrounded by light reflectors (not shown).
With this radiation detector RDA, the light guide LA is formed of an optically transparent material defining numerous slits MA of predetermined depths cut by a dicing saw or wire saw. The slits MA have optical elements (e.g. light reflecting elements or light transmitting elements) inserted therein. The slits MA have larger lengths from inner to outer positions of the light guide LA. This construction adjusts quantities of light from the scintillators S distributed to the four photomultiplier tubes K1–K4 to discriminate positions of incidence of gamma rays.
The conventional radiation detector RDA noted above has the following drawbacks.
The radiation detector RDA is a high-resolution detector using the scintillators S of high sensitivity as proposed in recent years, and the scintillator array SA has far more scintillators than the scintillator array of an earlier detector. Consequently, each scintillator S has a smaller section than a scintillator in the earlier detector. Generally, the smaller scintillators S provide, by absorption or diffusion, the lower probability of photons produced inside moving into the light guide LA. This reduces the capability of discriminating, and thus detecting, positions of incidence of gamma rays.
Further, because of limitations imposed by the shape of photomultiplier tubes K1–K4 and the shape of scintillators S, the X-direction (see FIG. 1) and Y-direction (see FIG. 1) are not necessarily in optically the same spatial relationship. It is therefore difficult to select widths of the light guide LA. Specifically, in the conventional example using two sets of 2-channel built-in photomultiplier tubes of 1-inch square, i.e. the four photomultiplier tubes K1–K4, the light guide LA must have a thickness of at least 10 mm in order to divide a 50 mm into ten in the Y-direction, and a thickness not exceeding 4 mm to divide a 25 mm into nine in the X-direction. That is, in order to distribute light emitted from the scintillators S to the photomultiplier tubes K1–K4 through the light guide LA, the light guide LA must have a thickness corresponding to the scintillators S or widths in the X- and Y-directions of the light guide LA. Thus, a reduction in size of each scintillator S causes a conflict between required specifications in the X- and Y-directions of the thickness of the light guide LA.
From the manufacturing point of view, a high accuracy of finishing is required particularly for the light guide LA optically combined to the scintillator array SA in order not to lower light transmission efficiency, and at the same time it is necessary to make the width of slits MA as small as possible. However, the accuracy of finishing the light guide LA is low where, as in the above conventional example, the radiation detector RDA is manufactured by cutting an optically transparent material with a dicing saw or wire saw to form the slits MA. Furthermore, the slits MA inevitably have coarse surfaces and have large widths. The light guide LA in the conventional example is manufactured by cutting the material into a plurality of parts with the dicing saw or wire saw to form the slits MA, and then combining the parts. This assembling operation is complicated and results in a cost increase.
When suitable light reflecting elements DA are inserted in the slits MA after the above process, gaps are formed between the reflecting elements DA and slits MA, thereby lowering reflection efficiency also. As these factors reduce output by incident gamma rays to make an accurate discrimination of positions impossible, an overall image quality will also deteriorate.
More particularly, a reduced discriminating ability results in a reduction in resolution. Where such radiation detector RDA is used in a medical diagnostic apparatus such as a PET apparatus or SPECT apparatus, images obtained by the apparatus will have poor quality. When a site of interest is a tumor, for example, the tumor may not be accurately outputted on the image.
Further, with the conventional processing method, it is difficult to form slits MA inclined relative to the direction of arrangement of the scintillator array SA.